Method of electrolytically pointing wires



Oct. 25, 1955 s. H. R. VISSER 2,721,838

METHOD OF ELECTROLYTICALLY POINTING WIRES Filed Oct. 10 1951 -|NVENTOR Simon Henricus R udolphusvisser Agent United States Patent' METHOD OF ELECTROLYTICALLY POINTING WIRES Simon Henricus RudolphusVisser, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee This invention relates to methods of electrolytically pointing wires and more particularly to pointing very thin wires which are used as pointed electrodes in electrode systems. Such electrode systems are generally referred to as crystal detectors or crystal amplifiers or transistors if two or more such electrodes are provided.

In a known method the wires are vertically suspended as an anode in a bath containing etching liquid, current being supplied until the wire becomes punctiform in the vicinity of the meniscus of the liquid. This phenomenon is a result of a decrease in the current density and the concentration of the etching liquid towards the boundary between air and liquid at the area of the wire. The parts of the wire provided deep in the bath are dissolved completely.

In order to obtain uniform results, it is necessary for the wire to be thoroughly wetted by the liquid so that the meniscus produced is stable. For this purpose, in electrolytically etching tungsten wires in a solution of potassium hydroxide, copper chloride has previously been added to the bath which involves several disadvantages. The resultant bath is not stable due to copper hydroxide flocking, thus requiring continuous cleansing of the liquid.

It is an object of this invention to provide an improved method of electrolytically pointing a wire which avoids the aforesaid disadvantage.

According to the invention, a finely-divided insulating material insoluble in the bath liquid is added to the wire to be etched in an electrolytic bath. The term finelydivided is to be understood to mean that the size of grain is smaller than microns, i. e. smaller than the radius of the point to be formed. This condition must be fulfilled to ensure uniform current density during etching.

The material must furthermore be divided and must not constitute a homogeneous, dense layer since it must be impregnated with the etching liquid.

Furthermore, the material must be insulating since it must not deviate the current from the surface to be etched. The material may be provided on the wire in the form of an envelope, but as an alternative it may be contained in the metal of the wire itself. The latter case the quantity of insulating material is preferably from 0.1% to 3% by weight of the metal.

When using the invention, the metal is removed by etching, the insulating material retaining its initial form. If the substance is mixed with the metal, it acquires the shape of a very fine skeleton. When provided as an envelope, it remains in the form of a thin tube. In either case the remainder of the insulating substance insures satisfactory and stable adherence of the liquid. The remainder of substance is readily broken oflr the Wire, and in most cases already when the wire is withdrawn from the bath.

Consequently, the insulating material must be insoluble in the etching liquid being used or at least dissolved less rapidly than the metal.

Very satisfactory results are obtained with tungsten wire to which from 0.15% to 0.5% by weight of thorium oxide has been added.

The invention will now be explained more fully with reference to an example and to the accompanying drawing in which:

Fig. 1 shows diagrammatically a device for electrolytically pointing a wire;

Figs. 2 and 3 show, on a greatly exaggerated scale, part of a wire at the area at which the point is formed, for two difierent forms of the wire to be etched. Y

The device comprises a container 1 in which a plurality of tungsten wires 3 are suspended by means of a pinch 2. The thickness of the wires is, for example, microns. The tungsten contains 0.25% of thorium oxide in a very finely-divided state.

The container furthermore contains a cathode 4, a 40% KOH solution being used as the etching liquid.

The bath is traversed by a current which is about 20 milliamps. per wire, the voltage being about 2 volts.

After a short time, the current will decrease, which indicates that the wires contained in the liquid are dissolved for the greater part. The wires then have the shape illustrated in Fig. 2. Each tungsten wire 3 has acquired a point 6 in the vicinity of the meniscus, a very loosely coherent skeleton 7 of thorium oxide being formed at the foot of the wire. This skeleton has a coherence suificient to ensure a constant form of the meniscus 5, but it breaks off with the slightest touch.

The moment at which the current is switched off is important for the shape of point 6. It is evident that the current must not be switched olf before the lower part of the wire has dissolved completely, whereas waiting too long results in point 6 becoming unduly obtuse. The correct moment may be ascertained by following the process through a microscope or by making tests. The correct moment once being known, the device may be automatized in a simple manner with the use of current and time relays.

Another method of obtaining such pointed wires consists in that a wire is provided with an insulating envelope 8 (see Fig. 3) for example by causing a thin layer of aluminium oxide or magnesium oxide to be deposited thereon by cataphoretic means. The etching treatment may subsequently take place in the manner above described. In this case a thin tube 9 of the oxide layer 8 is formed at the foot of the tungsten wire 3. The oxide layer also insures that the meniscus does not vary in shape during etching.

While the invention has thus been described with specific examples and applications other modifications therewith will be readily apparent to those skilled in the art without departing from the spirit and scope of the appended claims.

What I claim is:

l. A method of electrolytically pointing wire comprising the steps of uniting to at least the surface of the end of the wire a finely-divided porous electrically insulating material insoluble in a liquid of an electrolytic bath, suspending the wire as an anode in the electrolytic bath, and passing current through the bath until the end of the wire becomes punctiform in the vicinity of the meniscus formed by the bath liquid about the wire.

2. A method of electrolytically pointing wire comprising the steps of uniting to the surface of the wire a coating of finely divided porous electrically insulating material insoluble in a liquid of an electrolytic bath, suspending the wire as an anode in the electrolytic bath, and passing current through the bath until the end of the wire becomes punctiform in the vicinity of the meniscus formed by the bath liquid about the wire.

3. A method of electrolytically pointing wire comprising the steps of introducing into at least the surface of the wire finely-divided porous electrically insulating material insoluble in a liquid of the electrolytic bath, suspending the wire as an anode in the electrolytic bath, and passing current through the bath until the end of the wire becomes punctiformin the vicinity of the meniscus formed by the bath liquid about thewire.

4. A method of electrolytically pointing Wi1'6"COI11- prising the-steps of introducing 'intothe entire wire about 0.1 to 3% by weight of finely-divided porous electrically insulating material insoluble 'in a liquid of 'an electrolytic bath, suspending the wire as an anode in the electrolytic bath, and passing-current through the birth until the end of the Wire becomes punctiform in the vicinity of the meniscus formed-by the bath liquid about the wire.

'5. A method of electrolyticallypointing tungsten'wire comprising the-steps of introducing about 0.15 to 0.5% of 4 thorium oxide into the entire tungsten wire, suspending the wire as an anode in "an electrolytic bath, "and-passing current through the bath until the end of the wire becomes punctiform in the vicinity of the meniscus formed by the bath liquid about the wire.

References Cited in the file of this patent UNITED STATES PATENTS 2,018,470 Ruben Oct. 22, 1935 2,434,286 Pfann Jan. 13, 1948 2,545,866 Whitzel et al Mar. 20, 1951 2,628,936 Albano Feb. 17, 1953 FOREIGN PATENTS 322,118 Great Britain Dec. 5, 1929 

1. A METHOD OF ELECTROLYTICALLY POINTING WIRE COMPRISING THE STEPS OF UNITING TO AT LEAST THE SURFACE OF THE END OF THE WIRE A FINELY-DIVIDED POROUS ELECTICALLY INSULATING MATERIAL INSOLUBLE IN A LIQUID OF AN ELECTROLYTIC BATH, SUSPENDING THE WIRE AS AN ANODE IN THE ELECTROLYTIC BATH, AND PASSING CURRENT THROUGH THE BATH UNTIL THE END OF THE WIRE BECOMES PUNCTIFORM IN THE VICINITY OF THE MENISCUS FORMED BY THE BATH LIQUID ABOUT THE WIRE. 